1. Field of the Invention
The present invention generally relates to a method of sintering an injection-molded article of metallic or ceramic powder an more particularly, to a method of sintering an injection-molded article, for example, of stainless steel powder having a marked tendency to be oxidized.
2. Description of the Prior Art
Conventionally, a method of sintering an injection-molded article is often employed in producing an article which requires high density, high strength and high accuracy. The injection-molded article is generally made of raw material powder and organic binder.
In this method, the raw material powder of Fe-Ni, SUS (stainless steel: JIS G 4311), ceramics or the like is initially mixed and stirred with the organic binder and is injection-molded together therewith by an injection molding machine so that an injection-molded product or article formed in a desired configuration may be obtained. The majority of the binder is then dissolved and removed from the molded article by heating it, and thereafter, the molded article is sintered by being heated up to a sintering temperature of the raw material powder.
The raw material powder employed in this method is different from that to be sintered through a certain molding process other than the injection molding process. Since the raw material powder employed in this method is of minute powder comprising substantially spherical particles, whose diameter is less than 10 microns, it is sintered by using a method disclosed, for example, in Japanese Patent Laid-open Application (Tokkaisho) No. 57-123902.
More specifically, this sintering method comprises a debinderizing step of dissolving and removing the majority of the binder from the injection-molded article, a decarburizing step of dissolving and removing residual binder from the molded article, and a reducing and sintering step of sintering the molded article with an oxide formed during the removal of the binder being removed or reduced.
The decarburizing step and the reducing and sintering step are carried out under the atmospheric pressure within a certain atmosphere of mixed gas comprising inert gas (Ar gas or the like) and reducing gas (H.sub.2 gas or the like). During these steps, the molded article is heated while the dew point of the atmosphere within a furnace is controlled in compliance with the kind of molded article to be treated.
In the above described conventional method, since the molded article is treated within the atmosphere gas under the atmospheric pressure, heat loss through a furnace wall is relatively large, thus resulting disadvantageously in low heat efficiency. In treating a material, for example, SUS (stainless steel) including Cr of the like which has a marked tendency to be oxidized, it is necessary to keep the dew point of the atmosphere within the furnace at a temperature of approximately -60.degree. C. during the reducing and sintering step. This dew point of the atmosphere can be hardly maintained in the above descibed method to be executed within the atmosphere under the atmospheric pressure. Even if possible, such a method is disadvantageous in that much time is required for the treatment and high-performance equipment is inevitably needed at increased cost.
The present invention has been developed in view of the following points:
(1) an oxide film formed on the surface of the raw material powder should be limited to the minimum during the decarburizing step of removing the residual binder so that subsequent reducing and sintering step can be readily executed within a short time;
(2) the heating under reduced pressure raises the heat efficiency, and promotes pyrolysis of the binder and restrains the oxide from growing during the decarburizing step;
(3) through the heating under the reduced pressure, the reduction of the oxide is promoted during the reducing and sintering step; and
(4) the decarburizing and reducing ability of the atmosphere within the furnace is kept effectively when it is successively or intermittently exhausted.